On a touch screen display device, a user may perform an input operation by touching any point on the screen with a stylus pen or a finger. To this end, in addition to a plurality of pixels for displaying an image, the touch screen display device includes a plurality of touch sensing elements to sense a point which a user touches.
A drive signal and a data signal are applied to each pixel, and each touch sensing element senses a touch by a user and outputs a corresponding sensing signal. To this end, the touch screen display device includes a drive driving unit and a data driving unit for applying a drive signal and a data signal, and a sensing line signal processing circuit for processing an output signal of each touch sensing element that is output to a sensing line.
FIG. 1 is a view of a related art sensing line signal processing circuit of a touch screen. Referring to FIG. 1, a related art processing circuit includes sensing read circuit units 10A, 10B, . . . , 10N, a switch unit 20, and an A/D converter 30. Each sensing read circuit unit 10A to 10N includes a sensor capacitor Cs, a pre-charge switch PC, a readout switch RO, and an analog sensing channel 11,
The sensor capacitor Cs is connected to a plurality of sensing lines that are arranged on a touch screen panel in one direction, for example, in a vertical direction. A component Cp between a ground terminal and a pad around the sensor capacitor Cs is a parasitic capacitor.
The pre-charge switch PC is turned on in a pre-charge mode for a given time and thus a power supply terminal voltage VDD is pre-charged in the sensor capacitor Cs through the pre-charge switch PC.
Subsequently, the readout switch RO is turned on in a readout mode for a given time and thus the voltage charged in the sensor capacitor Cs is transferred to the analog sensing channel 11 through the readout switch RO.
However, if the sensor capacitor Cs is touched by a user on the touch screen panel, a distance between its electrode plates narrows and thus a capacitance changes. Accordingly, the voltage transferred from the sensor capacitor Cs to the analog sensing channel 11 decreases.
The analog sensing channel 11 integrates an input voltage to generate a corresponding touch sensing output voltage Vout1. In this case, the touch sensing output voltage Vout1 of the analog sensing channel 11 corresponds to a value that is obtained by dividing an input charge by a capacitance of a feedback capacitor CFB. That is, an integral value of the sensing line currents becomes an output charge.
As described above, an absolute value comparison technique is typically applied, in which a voltage of the sensor capacitor Cs connected to one touch line is compared with a reference voltage Vref at an operational amplifier OP of the analog sensing channel 11, and a corresponding touch sensing output voltage Vout1 is determined.
For example, a sensing read circuit unit 10A generates a touch sensing output voltage Vout1 of one touch line through these processes and generates touch sensing output voltages Vout2 to Voutn of the other touch lines through sensing read circuits 10B to 10N in the same way.
The switch unit 20 includes as many switches SW1 to SWn as the number of the sensing read circuit units 10A to 10N, and turns sequentially on them to sequentially transfer the touch sensing output voltages Vout2 to Voutn output from the sensing read circuit units 10A to 10N to the A/D converter 30.
The A/D converter 30 converts and outputs analog touch sensing output voltages Vout2 to Voutn input through these processes into digital signals.
A system control unit (not shown) identifies a vertical coordinate touched on the touch screen panel on the basis of the digital signal output from the A/D converter 30 and a horizontal coordinate on the basis of a signal detected through a drive line or a separately installed horizontal line to determine vertical and horizontal touch coordinates.
For example, if there are 100 drive lines and 100 sensing lines on the touch screen panel, a changed vertical coordinate signal can be output from the sensing read circuit connected to the 30th sensing line. Then, if the 50th drive line signal is applied, the X-axis coordinate is 50 and the Y-axis coordinate is 30. That is, the point where the 50th drive line intersects with the 30th sensing line is determined as a touched region.
However, if there is noise, such a general circuit has a limitation in that its operation is not stable. FIG. 2 is a graph of a result of how the touch sensing output voltage Vout is distorted if there is much noise when a touch is input.
If there is noise when the touch operation is carried, an output of the operational amplifier OP connected to the signal sensing line exceeds a normal operating range, and thus an output waveform may be distorted and it may be difficult to determine the input touch.
FIG. 3 is a view comparing levels of mutual capacitances if a normal touch is carried out, and FIG. 4 is a view comparing levels of mutual capacitances if noise is included when a touch is carried out.
In the case that driving lines are arranged in the X-axis direction and sensing lines are arranged in the Y-axis direction, if a normal touch signal is input, it is possible to identify a touched location because the mutual capacitance of the touched location is relatively small.
However, as shown in FIG. 4, if noise is included when a touch is carried out, it is difficult to read a mutual capacitance from an analog sensing channel that receives a signal from the sensing line. Noise can cause an output to be out of the range of voltages detectable from the analog sensing channel, thereby causing an abnormal mutual capacitance.